MD5 isn’t a particularly well regarded hashing function these days, but it’s still pretty ubiquitous.

SIP authentication, for the most part, still uses MD5 in the form of Message Digest Authentication,

If we were to take the password password and hash it using an online tool to generate MD5 Hashes we’d get “5f4dcc3b5aa765d61d8327deb882cf99”

If we hash password again with MD5 we’d get the same output – “5f4dcc3b5aa765d61d8327deb882cf99”,

The catch with this is if you put “5f4dcc3b5aa765d61d8327deb882cf99” into a search engine, Google immediately tells you it’s plain text value. That’s because the MD5 of password is always 5f4dcc3b5aa765d61d8327deb882cf99, hashing the same input phase “password” always results in the same output MD5 hash aka “response”.

By using Message Digest Authentication we introduce a “nonce” value and mix it (“salt”) with the SIP realm, username, password and request URI, to ensure that the response is different every time.

Let’s look at this example REGISTER flow:

We can see a REGISTER message has been sent by Bob to the SIP Server.

REGISTER sips:ss2.biloxi.example.com SIP/2.0    
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7    
Max-Forwards: 70    
From: Bob <sips:[email protected]>;tag=a73kszlfl    
To: Bob <sips:[email protected]>    
Call-ID: [email protected]    
CSeq: 1 REGISTER    
Contact: <sips:[email protected]>    
Content-Length: 0

The SIP Server has sent back a 401 Unauthorised message, but includes the WWW-Authenticate header field, from this, we can grab a Realm value, and a Nonce, which we’ll use to generate our response that we’ll send back.

 SIP/2.0 401 Unauthorized    
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7     ;received=192.0.2.201    
From: Bob <sips:[email protected]>;tag=a73kszlfl    
To: Bob <sips:[email protected]>;tag=1410948204    
Call-ID: [email protected]    
CSeq: 1 REGISTER    
WWW-Authenticate: Digest realm="atlanta.example.com", qop="auth",nonce="ea9c8e88df84f1cec4341ae6cbe5a359",     opaque="", stale=FALSE, algorithm=MD5    
Content-Length: 0

The formula for generating the response looks rather complex but really isn’t that bad.

HA1=MD5(username:realm:password)
HA2=MD5(method:digestURI)
response=MD5(HA1:nonce:HA2)

Let’s say in this case Bob’s password is “bobspassword”, let’s generate a response back to the server.

We know the username which is bob, the realm which is atlanta.example.com, digest URI is sips:biloxi.example.com, method is REGISTER and the password which is bobspassword. This seems like a lot to go through but all of these values, with the exception of the password, we just get from the 401 headers above.

So let’s generate the first part called HA1 using the formula HA1=MD5(username:realm:password), so let’s substitute this with our real values:
HA1 = MD5(bob:atlanta.example.com:bobspassword)
So if we drop bob:atlanta.example.com:bobspassword into our MD5 hasher and we get our HA1 hash and it it looks like 2da91700e1ef4f38df91500c8729d35f, so HA1 = 2da91700e1ef4f38df91500c8729d35f

Now onto the second part, we know the Method is REGISTER, and our digestURI is sips:biloxi.example.com
HA2=MD5(method:digestURI)
HA2=MD5(REGISTER:sips:biloxi.example.com)
Again, drop REGISTER:sips:biloxi.example.com into our MD5 hasher, and grab the output – 8f2d44a2696b3b3ed781d2f44375b3df
This means HA2 = 8f2d44a2696b3b3ed781d2f44375b3df

Finally we join HA1, the nonce and HA2 in one string and hash it:
Response = MD5(2da91700e1ef4f38df91500c8729d35f:ea9c8e88df84f1cec4341ae6cbe5a359:8f2d44a2696b3b3ed781d2f44375b3df)

Which gives us our final response of “bc2f51f99c2add3e9dfce04d43df0c6a”, so let’s see what happens when Bob sends this to the SIP Server.

REGISTER sips:ss2.biloxi.example.com SIP/2.0 
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92    
Max-Forwards: 70    
From: Bob <sips:[email protected]>;tag=ja743ks76zlflH    
To: Bob <sips:[email protected]>    
Call-ID: [email protected]    
CSeq: 2 REGISTER    
Contact: <sips:[email protected]>    
Authorization: Digest username="bob", realm="atlanta.example.com", nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="",     uri="sips:ss2.biloxi.example.com", response="bc2f51f99c2add3e9dfce04d43df0c6a"    
Content-Length: 0

SIP/2.0 200 OK
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92;received=192.0.2.201    
From: Bob <sips:[email protected]>;tag=ja743ks76zlflH
To: Bob <sips:[email protected]>;tag=37GkEhwl6
Call-ID: [email protected]    
CSeq: 2 REGISTER
Contact: <sips:[email protected]>;expires=3600
Content-Length: 0

There you have it, a 200 OK response and Bob is registered on biloxi.example.com.

Update 2021: Jason Murley has contributed a much more robust version of the code below, which is way better than what I’d made!

You can find his code here.

I’ve written a little tool in Python to generate the correct response based on the nonce and values associated with it:

import hashlib

nonce = 'ea9c8e88df84f1cec4341ae6cbe5a359'
realm = 'sips:biloxi.example.com'
password = 'bobspassword'
username    =   str("bob")
requesturi  =   str(s"ips:biloxi.example.com")
print("username: " + username)
print("nonce: " + nonce)
print("realm: " + realm)
print("password: " + password)
print("\n")

HA1str = username + ":" + realm + ":" + password
HA1enc = (hashlib.md5(HA1str.encode()).hexdigest())
print ("HA1 String: " + HA1str)
print ("HA1 Encrypted: " + HA1enc)
HA2str = "REGISTER:" + requesturi
HA2enc = (hashlib.md5(HA2str.encode()).hexdigest())

print ("HA2 String: " + HA2str)
print ("HA2 Encrypted: " + HA2enc)

responsestr = HA1enc + ":" + nonce + ":" + HA2enc
print("Response String: " + responsestr)
responseenc = str((hashlib.md5(responsestr.encode()).hexdigest()))
print("Response Encrypted" + responseenc)